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fa-fw"></i></a></div></div></nav><div id="post-info"><h1 class="post-title">JS异步函数小结</h1><div id="post-meta"><div class="meta-firstline"><span class="post-meta-date"><i class="far fa-calendar-alt fa-fw post-meta-icon"></i><span class="post-meta-label">发表于</span><time class="post-meta-date-created" datetime="2020-01-04T07:49:12.000Z" title="发表于 2020-01-04 15:49:12">2020-01-04</time><span class="post-meta-separator">|</span><i class="fas fa-history fa-fw post-meta-icon"></i><span class="post-meta-label">更新于</span><time class="post-meta-date-updated" datetime="2021-03-19T01:26:28.785Z" title="更新于 2021-03-19 09:26:28">2021-03-19</time></span><span class="post-meta-categories"><span class="post-meta-separator">|</span><i class="fas fa-inbox fa-fw post-meta-icon"></i><a class="post-meta-categories" href="/categories/Web%E5%89%8D%E7%AB%AF/">Web前端</a></span></div><div class="meta-secondline"><span class="post-meta-separator">|</span><span class="post-meta-wordcount"><i class="far fa-file-word fa-fw post-meta-icon"></i><span class="post-meta-label">字数总计:</span><span class="word-count">2.1k</span><span class="post-meta-separator">|</span><i class="far fa-clock fa-fw post-meta-icon"></i><span class="post-meta-label">阅读时长:</span><span>8分钟</span></span><span class="post-meta-separator">|</span><span class="post-meta-pv-cv" id="" data-flag-title="JS异步函数小结"><i class="far fa-eye fa-fw post-meta-icon"></i><span class="post-meta-label">阅读量:</span><span id="busuanzi_value_page_pv"></span></span></div></div></div></header><main class="layout" id="content-inner"><div id="post"><article class="post-content" id="article-container"><h1 id="JS的异步"><a href="#JS的异步" class="headerlink" title="JS的异步"></a>JS的异步</h1><p>JavaScript的执行环境是<strong>单线程的</strong>，对于http事件触发线程、浏览器事件触发线程、浏览器定时器等浏览器会单独开辟出一个异步线程处理，处理完毕后，加入任务队列，等待JS主线程调用执行。</p>
<p>例如：<br><figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="built_in">setTimeout</span>(<span class="function">()=&gt;</span><span class="built_in">console</span>.log(<span class="string">&#x27;触发了&#x27;</span>),<span class="number">0</span>);</span><br><span class="line"><span class="built_in">console</span>.log(<span class="string">&#x27;我先触发&#x27;</span>);</span><br><span class="line"></span><br><span class="line"><span class="comment">// 输出</span></span><br><span class="line"><span class="comment">/*</span></span><br><span class="line"><span class="comment">我先触发</span></span><br><span class="line"><span class="comment">触发了</span></span><br><span class="line"><span class="comment">*/</span></span><br></pre></td></tr></table></figure></p>
<p>虽然setTimeout()被设置为马上触发，但是setTimeout触发的异步任务需先放在任务队列中，等主线程中console()函数执行完毕后，再能被触发。</p>
<blockquote>
<p>JavaScript执行环境(浏览器)是从头到尾一行一行往下执行，但是遇到异步任务，先放入任务队列，等待主线程可以执行该任务，才被执行。</p>
</blockquote>
<p>详细的JS代码执行顺序可查看 <a target="_blank" rel="noopener" href="https://juejin.im/post/59e85eebf265da430d571f89">https://juejin.im/post/59e85eebf265da430d571f89</a></p>
<p>异步程序在JS代码中很常见，因为Web应用总归要与远方的服务器交互，请求数据，这个过程需要异步进行。否则，浏览器会一直卡住，直到结果请求完毕。</p>
<p>我们先看一下最简单的回调函数：setTimeout() 与 setsetInterval()，它们都是Windows对象的自带方法。</p>
<h2 id="setTimeout"><a href="#setTimeout" class="headerlink" title="setTimeout"></a>setTimeout</h2><p>setTimeout()表示指定一个异步程序，在规定的时间后执行，但不保证百分百执行。</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 2000ms后执行 console.log()</span></span><br><span class="line"><span class="comment">// 方法一</span></span><br><span class="line"><span class="built_in">setTimeout</span>(<span class="function">()=&gt;</span> <span class="built_in">console</span>.log(<span class="string">&quot;执行啦！&quot;</span>), <span class="number">2000</span>)</span><br><span class="line"></span><br><span class="line"><span class="comment">// 方法二</span></span><br><span class="line"><span class="keyword">let</span> innerFun = <span class="function"><span class="keyword">function</span>(<span class="params">test</span>)</span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(test);</span><br><span class="line">&#125;</span><br><span class="line"><span class="built_in">setTimeout</span>(innerFun, <span class="number">2000</span>, <span class="string">&#x27;执行啦！&#x27;</span>);</span><br></pre></td></tr></table></figure>
<p>如果前面有耗时的密集计算，哪怕setTimeout()设定了在很短的时间后执行，也要等主线程执行完毕，所以说 setTimeout()的延迟时间为理想时间。</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 预期1s后执行console.log()</span></span><br><span class="line"><span class="keyword">let</span> s = <span class="built_in">Date</span>.now();</span><br><span class="line"><span class="keyword">let</span> innerFun = <span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log((<span class="built_in">Date</span>.now()-s)/<span class="number">1000</span>);</span><br><span class="line">&#125;</span><br><span class="line"><span class="built_in">setTimeout</span>(innerFun, <span class="number">1000</span>);</span><br><span class="line"><span class="keyword">let</span> m = <span class="number">0</span>;</span><br><span class="line"><span class="keyword">for</span>(<span class="keyword">let</span> i=<span class="number">1</span>; i&lt;<span class="number">100000</span>;i++)&#123;</span><br><span class="line">    <span class="keyword">for</span>(<span class="keyword">let</span> j =<span class="number">0</span>; j&lt;i; j++)</span><br><span class="line">        m += j;</span><br><span class="line">&#125;</span><br><span class="line"><span class="comment">// 实际5.547后执行</span></span><br></pre></td></tr></table></figure>
<p>clearTimeout() 方法可取消由 setTimeout() 方法设置的定时操作。</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">let</span> timeout = <span class="built_in">setTimeout</span>(<span class="function">() =&gt;</span> <span class="built_in">console</span>.log(<span class="string">&quot;执行!&quot;</span>), <span class="number">1000</span>);</span><br><span class="line"><span class="built_in">clearTimeout</span>(timeout);</span><br></pre></td></tr></table></figure>
<h2 id="setInterval"><a href="#setInterval" class="headerlink" title="setInterval"></a>setInterval</h2><p>setInterval()跟setTimeout用法类似，只不过不是执行一次，而是如果不停止就永远重复执行。</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 每1000ms执行一次setInterval</span></span><br><span class="line"><span class="keyword">let</span> i =<span class="number">0</span>;</span><br><span class="line"><span class="keyword">let</span> interval = <span class="built_in">setInterval</span>(<span class="function">()=&gt;</span><span class="built_in">console</span>.log(i++), <span class="number">1000</span>)</span><br></pre></td></tr></table></figure>
<p>clearInterval()可以清除interval操作。</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="built_in">clearInterval</span>(interval);</span><br></pre></td></tr></table></figure>
<p>setInterval()指定的时间也是理想的时间，该时间包括程序本身的运行时间。如果程序本身运行时间大于指定时间，则setInterval()无法满足间隔时间定时运行。</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">let</span> s = <span class="built_in">Date</span>.now();</span><br><span class="line">innerFun = <span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">    <span class="keyword">let</span> m = <span class="number">0</span>;</span><br><span class="line">    <span class="keyword">for</span>(<span class="keyword">let</span> i=<span class="number">1</span>; i&lt;<span class="number">100000</span>;i++)&#123;</span><br><span class="line">        <span class="keyword">for</span>(<span class="keyword">let</span> j =<span class="number">0</span>; j&lt;i; j++)</span><br><span class="line">            m += j;</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="built_in">console</span>.log((<span class="built_in">Date</span>.now() - s)/<span class="number">1000</span>)</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="built_in">setInterval</span>(innerFun, <span class="number">1000</span>)</span><br><span class="line"><span class="comment">// 每次的运行时间都远远大于1s</span></span><br></pre></td></tr></table></figure>
<p>如果你想无论程序执行多长时间，每间隔固定时间都触发程序，你可以利用setTimeout()递归实现：</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">let</span> i = <span class="number">1</span>;</span><br><span class="line"><span class="keyword">let</span> run = <span class="built_in">setTimeout</span>(<span class="function"><span class="keyword">function</span> <span class="title">recur</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(i++);</span><br><span class="line">    <span class="built_in">setTimeout</span>(recur, <span class="number">1000</span>);</span><br><span class="line">&#125;, <span class="number">1000</span>);</span><br></pre></td></tr></table></figure>
<p>上面的程序可以保证每隔1000ms就执行console.log()方法，而不用管程序的执行时间。</p>
<blockquote>
<p>setTimeout() 与 setInterval()都是系统自带的基础的异步程序添加方法，可以时间指定延迟时间触发程序。</p>
</blockquote>
<p>旧版本JS的异步是依靠回调函数实现的，例如传统的AJAX：</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line">$.get(<span class="string">&#x27;ajax/text.html&#x27;</span>, <span class="function"><span class="keyword">function</span>(<span class="params">data</span>)</span>&#123;</span><br><span class="line">    <span class="comment">// doSomeThing</span></span><br><span class="line">&#125;)</span><br><span class="line"></span><br><span class="line">request.onreadystatechange = <span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">    <span class="keyword">if</span> (request.readyState === <span class="number">4</span>) &#123;</span><br><span class="line">        <span class="keyword">if</span> (request.status === <span class="number">200</span>) &#123;</span><br><span class="line">            <span class="keyword">return</span> success(request.responseText);</span><br><span class="line">        &#125; <span class="keyword">else</span> &#123;</span><br><span class="line">            <span class="keyword">return</span> fail(request.status);</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>但是回调函数之间往往存在依赖关系，即下一次回调函数的执行依赖于上一次回调函数的执行结果，然后，当这种依赖变得越来越多，代码的可读性就变得很差，此时称为“回调地狱”。</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// callback hall</span></span><br><span class="line">chooseToppings(<span class="function"><span class="keyword">function</span>(<span class="params">toppings</span>) </span>&#123;</span><br><span class="line">  placeOrder(toppings, <span class="function"><span class="keyword">function</span>(<span class="params">order</span>) </span>&#123;</span><br><span class="line">    collectOrder(order, <span class="function"><span class="keyword">function</span>(<span class="params">pizza</span>) </span>&#123;</span><br><span class="line">      eatPizza(pizza);</span><br><span class="line">    &#125;, failureCallback);</span><br><span class="line">  &#125;, failureCallback);</span><br><span class="line">&#125;, failureCallback)</span><br></pre></td></tr></table></figure>
<p>此时还有一个问题，就是对于每个回调函数，都要重复写一个对应的 catch 失败的函数，显然是非常冗余的，因此，ES6提出了<font color="#f07c82"><a target="_blank" rel="noopener" href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise">Promise</a></font>方法！</p>
<h1 id="Promise"><a href="#Promise" class="headerlink" title="Promise"></a>Promise</h1><p>Promise，英语单词，承诺，就像它的名称一样，Promise对象一定会实现它的诺言，也就是执行的结果无论成功与失败，都会给你一个答复。</p>
<p>Promise是一个构造器，可以生成Promise对象，是ES6异步编程的解决方案。</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">let</span> promise = <span class="keyword">new</span> <span class="built_in">Promise</span>(<span class="function"><span class="keyword">function</span>(<span class="params">resolve, reject</span>) </span>&#123;</span><br><span class="line">    <span class="keyword">if</span>(<span class="comment">/*异步执行成功*/</span>)&#123;</span><br><span class="line">        resolve(res);</span><br><span class="line">    &#125;<span class="keyword">else</span>&#123;</span><br><span class="line">        reject(<span class="keyword">new</span> <span class="built_in">Error</span>(<span class="string">&#x27;error&#x27;</span>));</span><br><span class="line">    &#125;</span><br><span class="line">&#125;)</span><br></pre></td></tr></table></figure>
<p>Promise在异步过程中一共有三个状态，<strong>pending</strong>等待结果状态,<strong>fullfilled</strong>的执行成功状态以及<strong>rejected</strong>拒绝状态。</p>
<p>三个状态之间转换关系为：</p>
<blockquote>
<p>pending状态 -&gt; fullfilled 状态<br>pending状态 -&gt; rejected 状态</p>
</blockquote>
<p>pending无论转向哪两个状态都是resloved(已完成)状态。</p>
<p>一般resolved状态表示fullfilled状态，这是习惯的写法。</p>
<h2 id="Promise-then"><a href="#Promise-then" class="headerlink" title="Promise.then()"></a>Promise.then()</h2><p>then()方法是Promise原型对象的方法，Promise.prototype.then()。该方法接受两个参数， promise.then(onFulfilled, onRejected)，一个是异步函数执行成功的处理函数，一个是执行失败的处理函数。但是错误处理函数最好在处理语句的最后用Promise.catch()方法捕获（后面会讲到），因为可提高代码的可读性。</p>
<p><strong>then()方法的onFulfilled传参要求如下：</strong></p>
<ol>
<li><strong>必须是函数，不是函数被忽略。</strong></li>
</ol>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">let</span> promise = <span class="keyword">new</span> <span class="built_in">Promise</span>(<span class="function">(<span class="params">resolve, reject</span>) =&gt;</span>resolve(<span class="string">&#x27;你好&#x27;</span>));</span><br><span class="line"></span><br><span class="line">promise.then(<span class="string">&#x27;JS&#x27;</span>).then(<span class="function">(<span class="params">result</span>) =&gt;</span><span class="built_in">console</span>.log(result));</span><br><span class="line"></span><br><span class="line"><span class="comment">// 返回 你好，最开始的JS被忽略</span></span><br></pre></td></tr></table></figure>
<ol>
<li>其参数必须是上个then()或Promise函数所返回。</li>
</ol>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">let</span> promise = <span class="built_in">Promise</span>.resolve(<span class="string">&#x27;Promise返回的&#x27;</span>);</span><br><span class="line"><span class="keyword">let</span> ordinal = <span class="function">()=&gt;</span><span class="string">&#x27;仅仅函数返回的&#x27;</span>;</span><br><span class="line">promise.then(<span class="function">(<span class="params">result</span>) =&gt;</span><span class="built_in">console</span>.log(result)).then(ordinal).then(<span class="function">(<span class="params">result</span>) =&gt;</span><span class="built_in">console</span>.log(result));</span><br><span class="line"></span><br><span class="line"><span class="comment">// Promise返回的</span></span><br><span class="line"><span class="comment">// 仅仅函数返回的</span></span><br></pre></td></tr></table></figure>
<ol>
<li>如果函数没有返回值，则返回 undefined</li>
</ol>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">let</span> ordinal = <span class="function">()=&gt;</span><span class="string">&#x27;仅仅函数返回的&#x27;</span>;</span><br><span class="line"><span class="built_in">Promise</span>.resolve().then(<span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">    ordinal();</span><br><span class="line">&#125;).then(<span class="function">(<span class="params">result</span>) =&gt;</span><span class="built_in">console</span>.log(result));</span><br><span class="line"><span class="comment">// undefined</span></span><br></pre></td></tr></table></figure>
<ol>
<li>then()方法只执行一次。</li>
</ol>
<p>由上规则的值，then()存在链式法则，一系列异步操作可以通过一系列的then()方法连接。</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line">chooseToppings()</span><br><span class="line">.then(<span class="function"><span class="params">toppings</span> =&gt;</span> placeOrder(toppings))</span><br><span class="line">.then(<span class="function"><span class="params">order</span> =&gt;</span> collectOrder(order))</span><br><span class="line">.then(<span class="function"><span class="params">pizza</span> =&gt;</span> eatPizza(pizza))</span><br><span class="line">.catch(failureCallback);</span><br><span class="line"></span><br><span class="line"><span class="comment">// 或者</span></span><br><span class="line">chooseToppings().then(placeOrder).then(collectOrder).then(eatPizza).catch(failureCallback);</span><br></pre></td></tr></table></figure>
<h2 id="Promise-catch"><a href="#Promise-catch" class="headerlink" title="Promise.catch"></a>Promise.catch</h2><p>catch()方法是Promise原型对象的方法，Promise.prototype.catch()，用来捕获错误。</p>
<blockquote>
<p>最佳实战：永远在then()调用链的最后使用，可以捕获异步操作的任意步骤出现的错误。</p>
</blockquote>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">chooseToppings().then(placeOrder).then(collectOrder).then(eatPizza).catch(failureCallback);</span><br></pre></td></tr></table></figure>
<h2 id="Promise-finally"><a href="#Promise-finally" class="headerlink" title="Promise.finally"></a>Promise.finally</h2><p>finally()方法是Promise原型对象的方法，Promise.prototype.finally()，用来最异步操作执行后（无论成功还是失败）都执行的代码。</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">chooseToppings().then(placeOrder).then(collectOrder).then(eatPizza).catch(failureCallback).finally(server.stop);</span><br></pre></td></tr></table></figure>
<h2 id="Promise-all"><a href="#Promise-all" class="headerlink" title="Promise.all"></a>Promise.all</h2><p>可以令多个Pormise并行执行，等所有的Promise都执行完毕，然后按照Promise的顺序返回一个由结果构成的Array。</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">let</span> p1 = <span class="keyword">new</span> <span class="built_in">Promise</span>(<span class="function">(<span class="params">resolve, reject</span>) =&gt;</span> &#123;</span><br><span class="line">  resolve(<span class="string">&#x27;成功了&#x27;</span>)</span><br><span class="line">&#125;)</span><br><span class="line"></span><br><span class="line"><span class="keyword">let</span> p2 = <span class="keyword">new</span> <span class="built_in">Promise</span>(<span class="function">(<span class="params">resolve, reject</span>) =&gt;</span> &#123;</span><br><span class="line">  resolve(<span class="string">&#x27;success&#x27;</span>)</span><br><span class="line">&#125;)</span><br><span class="line"></span><br><span class="line"><span class="keyword">let</span> p3 = Promse.reject(<span class="string">&#x27;失败&#x27;</span>)</span><br><span class="line"></span><br><span class="line"><span class="built_in">Promise</span>.all([p1, p2]).then(<span class="function">(<span class="params">result</span>) =&gt;</span> &#123;</span><br><span class="line">  <span class="built_in">console</span>.log(result)               <span class="comment">//[&#x27;成功了&#x27;, &#x27;success&#x27;]</span></span><br><span class="line">&#125;).catch(<span class="function">(<span class="params">error</span>) =&gt;</span> &#123;</span><br><span class="line">  <span class="built_in">console</span>.log(error)</span><br><span class="line">&#125;)</span><br><span class="line"></span><br><span class="line"><span class="built_in">Promise</span>.all([p1,p3,p2]).then(<span class="function">(<span class="params">result</span>) =&gt;</span> &#123;</span><br><span class="line">  <span class="built_in">console</span>.log(result)</span><br><span class="line">&#125;).catch(<span class="function">(<span class="params">error</span>) =&gt;</span> &#123;</span><br><span class="line">  <span class="built_in">console</span>.log(error)      <span class="comment">// 失败了，打出 &#x27;失败&#x27;</span></span><br><span class="line">&#125;)</span><br></pre></td></tr></table></figure>
<h2 id="Promise-resolve"><a href="#Promise-resolve" class="headerlink" title="Promise.resolve"></a>Promise.resolve</h2><p>Promise.resolve()将现有对象转为Promise对象。</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="built_in">Promise</span>.resolve(<span class="string">&#x27;foo&#x27;</span>)</span><br><span class="line"><span class="comment">// 等价于</span></span><br><span class="line"><span class="keyword">new</span> <span class="built_in">Promise</span>(<span class="function"><span class="params">resolve</span> =&gt;</span> resolve(<span class="string">&#x27;foo&#x27;</span>))</span><br></pre></td></tr></table></figure>
<p>Promise的参数分为四种情况:</p>
<ol>
<li><p>参数是Promise实例，则依旧返回Promise实例。</p>
</li>
<li><p>参数是一个 thenable 对象，则将该对象转为 Promise 对象，并立即执行。</p>
</li>
</ol>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">let</span> thenable = &#123;</span><br><span class="line">  then: <span class="function"><span class="keyword">function</span>(<span class="params">resolve, reject</span>) </span>&#123;</span><br><span class="line">    resolve(<span class="number">42</span>);</span><br><span class="line">  &#125;</span><br><span class="line">&#125;;</span><br><span class="line"></span><br><span class="line"><span class="keyword">let</span> p1 = <span class="built_in">Promise</span>.resolve(thenable);</span><br><span class="line">p1.then(<span class="function"><span class="keyword">function</span>(<span class="params">value</span>) </span>&#123;</span><br><span class="line">  <span class="built_in">console</span>.log(value);  <span class="comment">// 42</span></span><br><span class="line">&#125;);</span><br></pre></td></tr></table></figure>
<ol>
<li>如果参数是一个原始值，或者是一个不具有then方法的对象，则Promise.resolve方法返回一个新的 Promise 对象，状态为resolved。</li>
</ol>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">const</span> p = <span class="built_in">Promise</span>.resolve(<span class="string">&#x27;Hello&#x27;</span>);</span><br><span class="line"></span><br><span class="line">p.then(<span class="function"><span class="keyword">function</span> (<span class="params">s</span>)</span>&#123;</span><br><span class="line">  <span class="built_in">console</span>.log(s)</span><br><span class="line">&#125;);</span><br><span class="line"><span class="comment">// Hello</span></span><br></pre></td></tr></table></figure>
<ol>
<li>不带有任何参数</li>
</ol>
<p>Promise.resolve方法允许调用时不带参数，直接返回一个resolved状态的 Promise 对象。<br><figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="built_in">setTimeout</span>(<span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">  <span class="built_in">console</span>.log(<span class="string">&#x27;three&#x27;</span>);</span><br><span class="line">&#125;, <span class="number">0</span>);</span><br><span class="line"></span><br><span class="line"><span class="built_in">Promise</span>.resolve().then(<span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">  <span class="built_in">console</span>.log(<span class="string">&#x27;two&#x27;</span>);</span><br><span class="line">&#125;);</span><br><span class="line"></span><br><span class="line"><span class="built_in">console</span>.log(<span class="string">&#x27;one&#x27;</span>);</span><br><span class="line"></span><br><span class="line"><span class="comment">// one</span></span><br><span class="line"><span class="comment">// two</span></span><br><span class="line"><span class="comment">// three</span></span><br></pre></td></tr></table></figure></p>
<p><strong>注意</strong></p>
<p>立即resolve的 Promise 对象是在事件循环中当前宏任务的后面添加微任务，因此需要等当前宏任务结束后再执行。</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="built_in">Promise</span>.resolve().then(<span class="function">() =&gt;</span> <span class="built_in">console</span>.log(<span class="number">2</span>)).then(<span class="function">() =&gt;</span> <span class="built_in">console</span>.log(<span class="number">3</span>));</span><br><span class="line"><span class="built_in">console</span>.log(<span class="number">1</span>); <span class="comment">// 1, 2, 3</span></span><br></pre></td></tr></table></figure></article><div class="post-copyright"><div class="post-copyright__author"><span class="post-copyright-meta">文章作者: </span><span class="post-copyright-info"><a href="mailto:undefined">孙云增</a></span></div><div class="post-copyright__type"><span class="post-copyright-meta">文章链接: </span><span class="post-copyright-info"><a 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